Plastic optical materials are more lightweight, less brittle, and more easily dyeable than optical materials composed of inorganic materials. Numerous plastic resin materials have been applied to optical materials and have been increasingly required to have excellent physical properties.
Polythiourethane optical resins produced using polythiol compounds and isocyanate compounds are widely used as materials for optical lenses due to their excellent optical properties in terms of transparency, Abbe number, and transmittance, and physical properties in terms of tensile strength.
Korean Patent Application No. 10-1995-001404 discloses a method for preparing a polythiol compound. According to this method, epichlorohydrin is reacted with 2-mercaptoethanol in the presence of triethylamine to obtain a diol, and the diol is reacted with sodium sulfide to obtain a tetraol. The tetraol is reacted with thiourea in hydrochloric acid to prepare an isothiouronium salt. Subsequently, rearrangement takes place to form a tetraisothiouronium salt. The tetraisothiouronium salt is hydrolyzed by the addition of aqueous ammonia to obtain the desired polythiol compound.
The patent application discloses another method for preparing a polythiol compound. According to this method, epichlorohydrin is reacted with thioglycerol in the presence of triethylamine to obtain a triol. Then, the reaction liquid is reacted with thionyl chloride to obtain a chlorinated derivative. At this moment, rearrangement takes place partially to form an isomer mixture tetrachloride. Then, the isomer mixture tetrachloride is reacted with thiourea to form an isothiouronium salt, to which hydrazine hydrate is added to hydrolyze the salt to obtain the desired polythiol.
Despite the advantages of simple reaction routes and low costs, the conventional methods generate large quantities of by-products and cause non-uniform colors of polythiol compounds. The by-products deteriorate the heat resistance of optical lenses, limiting the application of the polythiol compounds to resins for optical lenses.